The fundamental aspect of an HRM is to utilize a photochemical phenomenon wherein the light harvesting dye absorbs light, reacts with the polymerized matrix, and alters the index of refraction. These induced refractive index modulations result in phase holograms with high diffraction efficiency and wavelength and angular selectivity. The covalent reaction of the light harvesting dye with poly(methyl methacrylate) is known (See, for example, A. V. Veniaminov and H. Sillescu, Macromolecules, 32, 1828–1837, 1999). Previous HRMs are well known, but the HRM closest to the subject invention is limited to a poly(methyl methacrylate) (PMMA) polymer and a light harvesting dye, 9,10-phenanthrenequinone composite.
For example, A. Popov et al. (A. P. Popov, A. V. Veniaminov, Y. N. Sedunov, SPIE, 2215, 64, 1994) describe a general method of fabricating a 6 to 8 mm thick HRM having a gradient distribution of the 9,10-phenanthrenequinone dye in the PMMA matrix across the material's thickness.
This variation of the dye concentration as taught by Popov et al., is achieved by exposing each surface to a mercury lamp light filtered in such a way that the transmission maximum coincides with having a wavelength within the absorption profile of 9,10-phenanthrenequinone dye. As the light propagates through the HRM, its intensity falls exponentially with the penetration depth in accordance with the Lambert-Beer law. The accompanying photoinduced effect, a reaction between the dye and the polymer matrix, decreases. Thereby, unreacted dye is located toward the center of the HRM's cross-section.
In the same publication, Popov et al. describe another method of fabricating a thick HRM with a gradient distribution of the 9,10-phenanthrenequinone dye in a PMMA matrix. In this method, the initial 100 micrometers thick layer of PMMA polymer is doped with 10 wt % of 9,10-phenanthrenequinone, which was prepared from a dichloroethane solution. The dried film was then placed between two 3 mm thick pure PMMA slabs and the entire assembly pressed together and heated to accelerate dye diffusion from the center layer to outside layers. The diffusion into the PMMA slabs depends on the temperature. In most instances, the elevated temperature necessary to achieve reasonable diffusion rates exceeds the PMMA's glass transition temperature. Obviously, this result is not desired.
Likewise, B. Ludman et al. (J. E. Ludman, N. O. Reinhard, I. V. Semenova, Yu. L. Korzinin, and S. M. Shahriar, SPIE, 2532, 481, 1995) describe the use of an HRM consisting of 0.5 to 5 wt % of 9,10-phenanthrenequinone in a PMMA matrix. This HRM has similar problems of Popov et al.
Similarly, C. Steckman et al. (G. J. Steckman, I. Solomatine, G. Zou and D. Psaltis, Opt. Lett., 23, 1310, 1998) describe the preparation of a 1 to 5 mm HRM comprising 0.7 wt % of 9,10-phenanthrenequinone dye dissolved in a PMMA matrix. To prepare such material, a solution of the dye, a polymerization initiator, and methyl methacrylate, is poured into molds and allowed to polymerize in a pressure chamber at elevated temperatures.
A problem with these prior references is that the PMMA has a relatively low glass transition which can lead to distortions after light exposure. Another problem is that post exposure treatment at elevated temperatures (around and above the glass transition temperature), significantly reduces the photoinduced index modulation by the diffusion of the photoproducts and, consequently, the strength of the holograms substantially decreases. Another problem relates to the low number of reactive sites in the polymer matrix during holographic recording. Yet another problem involves the limited chemical inertness of the PMMA matrix toward common chemical agents such as alcohols, acetone, chloroform, benzene and others.